Several processes have been proposed for recovering metals or preparing metal products from aqueous solutions containing dissolved metals. Such solutions can result from a variety of sources, including industrial and mining operations. Often, such aqueous solutions result from a leach process to dissolve metals from a solid material.
Many processes have been proposed using liquid-liquid extraction techniques to extract one or more dissolved metals into an immiscible organic phase. Some processes involve several solvent extraction steps to selectively recover metals from the aqueous solution. For example, U.S. Pat. No. 3,988,224 by Barriere et al., issued Oct. 26, 1976, discusses a method for extracting metals from submarine nodules which involves at least three solvent extraction steps.
Solvent extraction procedures are expensive and often difficult to operate. Difficulty and expense generally increase when greater selectivity of extraction is required. Therefore, processes minimizing the number of solvent extraction steps, or requiring lower selectivity of extraction, are desirable.
In addition to extracting a desired metal from a solution, solvent extraction can also be used to extract undesirable contaminants from a solution leaving the desired metal behind in the raffinate. For example, U.S. Pat. No. 3,988,224, noted previously, discusses solvent extraction of some metal impurities from an aqueous chloride solution prior to recovering a metallic nickel product from the solution by electrolysis. Contaminants are extracted by contacting the aqueous solution with an organic phase including a nickel salt of di-(2-ethylhexyl) phosphoric acid.
Removal of contaminants is especially important for the preparation of metal salt products from an aqueous solution. In the process discussed in U.S. Pat. No. 3,988,224, the solution, even after removal of impurities, still contains significant quantities of dissolved magnesium contaminant in addition to the dissolved nickel to be recovered as product by electrolysis. The presence of significant quantities of a residual contaminant, such as magnesium, could create significant problems during recovery of many products from a solution, such as, for example, recovery of a metal salt product.
As noted, one common source of aqueous solutions containing dissolved metals is from leaching operations to dissolve metals from solid materials. Relatively little attention has been given, however, to processes for leaching and recovering metals from spent catalyst. A significant amount of catalyst, often containing valuable metals, is consumed by industry every year. Much of this spent catalyst is disposed of, resulting in significant waste disposal problems as well as the loss of valuable metals.
One valuable metal found in a variety of catalysts is nickel. Nickel catalysts, for example, are used in numerous industrial processes, including hydrogenation of oils, such as vegetable oils. Such nickel catalysts represent a significant source for potential nickel recovery. U.S. Pat. No. 4,415,541 by Melin, issued Nov. 15, 1983, discusses recovering nickel from a spent fat hardening catalyst. Nickel from the catalyst is leached into a sulfate solution, for recovery of the nickel therefrom. U.S. Pat. No. 4,415,541, however, provides no method for recovering a nickel product from the nickel sulfate solution and does not address the problem of metal contaminants which are frequently found in leach liquors.
Needs exist for improved processes for recovering metals and metal products from aqueous solutions containing dissolved metals, and for recovering metal salt products in particular. A need also exists for an efficient process for recovering metal values from spent catalysts, and particularly for recovering nickel values.